The response of a pathogen to the changing environments faced during the course of infection is transcriptionally regulated. Dr. Groisman is investigating the PhoP regulatory network which controls how S. typhimurium, a facultative intracellular pathogen, survives and multiplies within macrophages. PhoP is responsible for expression of loci essential for virulence in mice, intramacrophage survival and resistance to host defense systems (in particular the defensin peptide antibiotics). This network includes the regulator of transcription PhoP, the putative PhoP kinase known as PhoZ and the targets of PhoP activation and repression essential for pathogenesis. He proposes studies to explore the molecular basis for the avirulence phenotype of phoP and phoZ mutants in an effort to understand the role of the factors required for Salmonella pathogenesis. The specific goals are as follows: 1) Identification and characterization of the targets of PhoP regulation involved in virulence. To date, only 5 out of ca. 40 PhoP-regulated loci have been identified; only one of them seems to have a role in virulence. In order to understand the role of PhoP in Salmonella pathogenesis, the investigator will undertake several genetic approaches to identify the PhoP-activated and - repressed genes required for Salmonella pathogenesis. Strains harboring mutations in PhoP-regulated genes will be evaluated for virulence phenotypes which are defective in phoP- Salmonella: mouse virulence, intramacrophage survival and resistance to microbicidal peptides. A molecular genetic characterization (identification, mapping, cloning and DNA sequencing) will be performed on those mutants showing virulence defects. 2) Transcriptional regulation by PhoP. The PhoP-PhoZ pair is a member of the family of two-component regulators that control gene expression in response to environmental signals. To characterize the role of PhoP and PhoZ in control of virulence functions, Dr. Groisman will investigate the signals that modulate their activity, and pursue the purification of these proteins in order to test the postulated DNA binding properties of PhoP and the phosphotransferase (kinase) activities of PhoZ. The proposed studies should be an important step in the discovery of novel virulence genes, the identification of the physiological signals to which Salmonella responds to during infection and the understanding in molecular terms of the mechanism of PhoP regulation. The hypothesis that resistance to host defense peptides is necessary for Salmonella pathogenesis will also be investigated.